You might think that in terms of sound, as opposed to facilities and UI design, the only significant difference between plugin equalisers is whether or not they are clean, digital equalisers or whether they attempt to imitate the artefacts of an analogue EQ.
This is broadly true but there is an issue affecting some digital filters which can affect the sound, that is the much misunderstood issue of EQ “cramping”.
What Is EQ Cramping?
Digital filters are for the most part identical to their analogue counterparts on which they are based. However one of the way in which they can differ is when they are carried over to the digital domain by means of the ‘bilinear transform"‘. As long as the sampling rate exceeds the frequencies of interest by a comfortable margin, the bilinear transform provides a very accurate way to design a digital filter.
The problem with the bilinear transform only presents itself with certain filter types and only when the filter is applied at the top end where some of the cut or boost would pass beyond half the sampling frequency - the Nyquist frequency which represents the highest frequency which can be reproduced by the system at that sampling rate.
What Difference Does Cramping Make?
A consequence of using the bilinear transform is that the amplitude must be at unity at the Nyquist frequency. Because of this the symmetrical shape of bell filters applied at the high end will be distorted, squashing the slope of the bell above the centre frequency more and more as the centre frequency is raised.
This directly affects the sound. Your assumption about by how much a wide high-mid boost with a peaking filter at 12KHz would be boosting 16KHz could be very wrong and there are consequences in the phase response to be considered too.
How Common Is EQ Cramping?
Well designed, premium EQs avoid these issues. Internal over-sampling in one way to achieve this and while this was definitely an issue with early digital EQ’s, it’s less common than it used to be. However the only thing which really matters to you is whether the EQs you use display cramping. Rather than research it online you could always download a copy of Plugin Doctor and try for yourself. Watch the short video above to see how simple it is to set and for a brilliant, succinct but thorough explanation of EQ cramping watch Dan Worrall’s video “Don’t Cramp My EQ Style” below.
DDFM Plugin Doctor
If you are curious about your plugins and want to dive deeper into their performance consider buying a copy of DDFM’s excellent Plugin Doctor. It’s inexpensive and there is a restricted demo with which you can test for EQ cramping availble for free.